Printing machines of the kind described for example in Keller et al. U.S. Pat. No. 5,833,377 utilize a thermal transfer “inked” ribbon wound upon a core, the core in turn being mounted upon a spindle of the machine (see for example FIG. 4 of the patent). The spindle may have a plurality of longitudinal grooves and/or walls that engage corresponding ribs on the inside of the core to lock the core against relative rotation on the spindle. The spindle and core may also have cooperating means for limiting the depth to which the spindle can be inserted into the core, and for maintaining the core in its fully mounted position. The limiting and securing means described by Keller et al. take the form of a ramp and stop face, centrally located on the ribs of the core, which coact, respectively, with an abutment and a resilient spring finger disposed in a groove of the spindle.